Modern gun systems consist of a gun tube and a “pointing device” that orients the gun tube in three dimensional space, allowing the gun tube to be aimed in a desired direction. FIGS. 1A, 1B, and 1C, are block diagrams of a side view, a top view, and a front view respectively, of a modern gun system 100.
FIG. 1A shows the gun system 100 with a gun tube 102 having a muzzle 104, a breech 106, a projectile 108, a firing mechanism 110, and a pointing device 112. The gun system 100 may be used to fire one or more projectiles, such as the projectile 108. A soldier or other user of the gun system 100 may insert the projectile 108 into the firing mechanism 110, typically via the breech 106 of the gun tube 102, in the case of breech loaded weapons, or into the muzzle 104 in the case of muzzle loaded weapons (such as mortars). The soldier may aim the gun in a desired direction using the pointing device 112. The soldier may fire the gun system 100 causing the projectile 108 to leave the gun system 100 via the muzzle 104 to travel on a trajectory. After traveling on a trajectory, the projectile 108 impacts at an impact location.
The term “boresighting”, as used herein, is the procedure of aligning a pointing device of a gun system with the gun tube to a desired degree of accuracy and precision. Boresighting typically is performed both to initially establish alignment between the pointing device and the gun tube and periodically thereafter to ensure the pointing device is within its specified design performance.
Traditionally, both artillery and mortar weapons are boresighted using the Distant Aiming Point technique. This technique involves pointing the gun tube at some distant object by some independent means (actually sighting through the gun bore or employing some other device). Then, an aiming system, such as an optical sight or an electronic pointing device, is aimed at the same distant object. The “distant object” requirement reduces parallax errors that arise because the aiming system and gun tube do not share the same line of sight.
The orientation of the gun system 100 in three dimensional space may be specified in terms of three angles: an “elevation”, an “azimuth”, and a “roll”. The elevation of a gun system is the angle between a horizontal plane having the gravity vector as a surface normal and an axis of a gun tube of the gun system. FIG. 1A shows an elevation 120 of E° for the gun system 100. FIG. 1A depicts the elevation 120 as a dashed line indicating the angle between a horizontal plane 122 and a gun-tube axis 124 of the gun tube 102 of the gun system 100. The elevation may be expressed in angular units such as degrees, radians, or as a quadrant elevation (QE). The QE may be expressed in terms of any units of angular measure, such as degrees, radians, or “mils”. (There are 6,400 mils of arc in a circle; for example, a QE of 800 mils corresponds to a 45° angle.)
The azimuth indicates a direction of fire for the weapon system (i.e., the direction of the gun-tube axis 124) expressed as an angle from a reference direction, such as true north measured in the horizontal plane 122. FIG. 1B indicates an azimuth 140 of A°. FIG. 1B depicts the azimuth 140 as a dashed line, indicated with respect to a reference direction 130 and a direction of a gun tube 124. The azimuth may be expressed in terms of any units of angular measure, such as degrees, radians, or mils. This component of gun-tube orientation may be referred to as “deflection”, which is an angle measured with respect to an arbitrary reference direction having a known relationship to a global reference direction such as true north.
The roll indicates an angle of rotation about the gun-tube axis 124 of the gun tube 102. FIG. 1C shows a FIG. 1C shows a cross-section of the gun tube 102 in a grey color with a roll 150 of R°, as well as an inner diameter (ID) 160 of the gun tube, an outer diameter (OD) 170, and a vertical centerline 180 of the gun tube. As shown on FIG. 1C, the inner diameter 160 is the diameter of the gun tube bore and the outer diameter 170 is the diameter of the gun tube 102. The vertical centerline 180 is defined to be the tube diameter that lies in the plane that also contains the gravity vector and the gun-tube axis 124. Typically, the roll and the elevation are defined with respect to a gravity vector pointing toward the center of the earth. However, the azimuth is typically defined with respect to a geodetic grid and a horizontal plane specified by the gravity vector.
As modern gun tubes, such as the gun tube 102, are typically symmetric about the gun-tube axis 124, the roll component of tube orientation may or may not be specified as part of the target of the gun system 100. However, some roll component is typically assigned for the processes of both boresighting a gun system 100 and the subsequent determination of the pointing accuracy of the pointing device 112. A roll component is typically assigned because roll in one frame-of-reference (e.g., the gun tube 102) affects the azimuth and elevation in a second frame-of-reference (e.g., the pointing device 112).